convert FN_PARAM_MOVE_COUNT from SfxInt16 to SfxInt32

so that we can convert surrounding code from xub_StrLen to sal_Int32

Change-Id: Ib779c82a71405f855cfe72bcde2511e8153445f0
Reviewed-on: https://gerrit.libreoffice.org/6896
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>